Neurodegeneration is very often seen in patients with human immunodeficiency virus type I (HIV-1). The HIV-1 envelope protein gp120 has been implicated in the neurotoxicity caused by HIV-1, however, little is known about the molecular mechanism(s) of gp120 neurotoxicity. We have used, as an experimental model to study these mechanisms, primary culture of cerebellar granule cell neurons obtained from 8-day-old rats. These neurons undergo cell death when exposed to glutamate or other neurotoxic agents. gp120-induced a time-and concentration-dependent neurotoxicity as determined by in situ TUNEL and MTT assay. In fact, exposure of these cultures to 200 pM-20 nM of gp120 for 24 hr induced apoptosis in at least 80 percent of neurons. Moreover, the morphological appearance of these neurons was altered by gp120 such that the majority of these neurons showed short neuronal processes (both neurite and dendrites). Thus, gp120 induces a form of neuronal atrophy, which mimics that observed in young patients infected with the HIV. Neurotrophic factors have been shown to prevent apoptosis and neuronal atrophy caused by neurotoxic agents. In particular, brain derived neurotrophic factor (BDNF) has been shown to prevent glutamate toxicity in granule cells as well as in other neuronal populations. Thus, the first hypothesis that we will test is whether BDNF could limit gp120 neurotoxicity. Preliminary data in support of this hypothesis have been obtained by showing that exposure of cerebellar granule cells to BDNF prevented gp120 neurotoxicity. These findings give exciting hints to the potential mechanisms of gp120 toxicity in vivo and allowed us to formulate our overall hypotheses that gp120 may reduce synthesis and release of BDNF and therefore renders neurons more vulnerable to glutamate. As preliminary data to support this hypothesis, we show that gp120 decreases BDNF synthesis in cerebellar granule cells. However, it remains to establish whether such mechanism is operative in vivo as well. RNase protection assay, two-site immunoassay and trkB tyrosine phosphorylation will be used to document BDNF biosynthesis/activity in cerebellar granule cells in response to various concentrations of gp120. We will also use an in vivo model to provide direct evidence of the relevance of our in vitro findings. Gp120 will be injected in selected areas of the brain in postnatal and adult rats, as well as in wild type and BDNF heterozygous null (+/-) mice to further establish a direct relationship between availability of BDNF and gp120 neuronal toxicity. These studies are aimed at obtaining more information on the potential role of the neurotrophins in limiting gp120 toxicity and consequently on neuronal plasticity. Results from these studies will provide i) new insights into the molecular mechanisms whereby HIV causes neuronal degeneration and ii) the significance of endogenous neurotrophins in the development of new therapeutic approach for HIV-1 related neurodegenerative diseases.